The present invention relates to cooling the stator cores of large dynamoelectric machines, and more particularly to improved cooling of the end regions of such cores.
Dynamoelectric machines, such as large generators, have stator cores which are built up of laminations in order to reduce the eddy current loss in the core, but substantial hysteresis and eddy current losses occur and the resulting heat must be dissipated to keep the temperature rise within the required limits. This is usually done by dividing the laminations into packs which are spaced apart axially of the core to form radial vents for the circulation of coolant gas through the core.
These radial vents are formed by means of vent plates which are placed at intervals in the laminated core to space adjacent laminations apart and thus to form the vents between packs of tightly-clamped laminations. The conventional vent plates commonly used for this purpose consist of punchings which are identical in configuration to the punchings or laminations of which the core is built and provided with spacing fingers. The fingers are thin sheet metal members extending generally radially of the vent plate and projecting perpendicularly to the plane of the plate. The fingers are typically about 0.125 inch in height and are riveted to the plate. When such a vent plate is incorporated in a stack of laminations, the projecting fingers space the adjacent laminations apart to form a radial duct equal in width to the height of the fingers.
These conventional vent plates operate satisfactorily but involve certain problems due to unavoidable manufacturing variations. The fingers are produced by dies from thin sheet metal and vary slightly in dimensions and shape, and may also show a slight taper from one end to the other. The total thickness of the plate and finger at the points where the fingers are riveted to the plates can also vary from one place to another. These variations are unavoidable and while they are quite small if proper quality control is maintained, nevertheless they are sufficient to cause problems of mechanical stability in some cases and may affect the gas flow through the radial vents.
The problem of cooling the stator core is particularly difficult in the core end regions of large machines such as turbine generators. In large synchronous machines, the currents in the end turn portions of the rotor winding and in the end portions of the stator windings have magnetic fields which combine to produce an axially-directed magnetic flux. This axial flux enters the end of the stator core in a direction generally perpendicular to the core laminations and causes relatively large eddy currents in the end regions of the core since the core is not laminated in a direction to minimize these currents. The corresponding losses may be quite large and often cause excessive heating in the end regions of the core.
When conventional vent plates as described above are used in the end regions of a large stator core, it is not possible to provide radial vents close to the finger plates which clamp the core laminations. The mechanical stability problem discussed above makes it necessary to have a pack of laminations on each side of a conventional vent plate to bridge the spacing fingers. The use of bridging laminations in this way provides sufficient flexibility or resilience in the core, even when the laminations are tightly clamped, to enable the structure to yield enough to distribute the load and thus compensate for the unavoidable small manufacturing variations mentioned above. At each end of the stator core, a solid finger plate bears against the core and is engaged by a clamping plate to apply relatively heavy axial clamping pressure to the entire stack of core laminations. These finger plates and the stator core laminations tend to become quite hot because of the axial flux mentioned above, to which they are exposed, and because of heat conducted from the core itself, and cooling of this region of the stator core has been very difficult. Conventional vent plates cannot be placed directly adjacent the finger plates to provide radial ducts for coolant gas because of the mechanical stability problem discussed above, but must have a pack of laminations of substantial thickness on each side to properly distribute the load. This necessarily spaces the closest radial duct a considerable distance from the finger plate and stator core end laminations so that they are not effectively cooled.